This invention relates to xerography and, more particularly relates to compositions and methods for making bias transfer rolls and belts for transferring xerographic toner images from one support surface to another.
In conventional xerography a photosensitive plate, which consists of a photoconductive coating placed over a conductive backing is charged uniformly and the charged plate is then exposed to a light image of an original. Under the influence of the light image the charge on the plate is selectively dissipated to record the original input information on the plate in the form of a latent electrostatic image. The latent image is developed, or made visible, by applying oppositely charged toner particles to the plate surface in a manner so that the toner particles are attracted into the imaged areas. The developed images are generally transferred from the photoconductor to a final support material, such as paper or the like, and affixed thereto to form a permanent record of the original.
Image transfer from the photoconductor to the final support material may be accomplished by means of a corona induction using a corona generator, or it may be accomplished by a roller or belt electrode biased to a certain potential, such electrode being referred to as a bias transfer member (roll or belt). The corotron system is relatively simple, but the charges deposited by the corotron electrostatically tack the final support material, such as paper, to the original toner support, such as, the photoconductor, in addition to creating the desired electric field affecting transfer of the toner to the paper. The strong attraction between the paper and the original toner support makes it mechanically difficult to separate or detack the two supports.
Transfer of developed images from the photoconductor to the final support material with the aid of a biased transfer member is not well known in the art, and such a member generally avoids severe tacking problems which are encountered when the corona induction system is utilized. Bias transfer members are well known in the art. A bias transfer roll is disclosed by Fitch in U.S. Pat. No. 2,807,233 where a metal roll coated with a resilient coating having a resistivity of about 10.sup.6 to 10.sup.8 ohm cm is used as a bias transfer member. Shelffo in U.S. Pat. No. 3,520,604 suggests that in order to create the proper environment for the duplicating mode, a transfer roll is used and is made of a conductive rubber having a resistivity in the range of from about 10.sup.11 to about 10.sup.16 ohm cm. A bias transfer member, that is, a member for electrically cooperating with a conductive support surface to attract electrically, charged particles from the support surface towards the member, is described by Dolcimascolo et al in U.S. Pat. No. 3,702,482. In Dolcimascolo et al, the bias transfer member has a conductive substrate for supporting a bias potential thereon, an intermediate blanket placed in contact with the substrate having an electrical resistivity capable of readily transmitting the bias potential on the substrate to the outer periphery of the blanket and a relatively thin outer coating placed over the blanket having an electrical resistivity to minimize ionization of the atmosphere when the transferred member is placed in electrical cooperation with the image support surface and provides a good toner release property enabling the device to be cleaned of the toner. A typical material for the relatively thin outer coating in accordance with Dolcimascolo et al, is one formulated of a material capable of providing a relatively smooth surface exhibiting relatively good mechanical release properties in respect to the toner materials employed. One such material is a polyurethane material manufactured by the duPont Company under the tradename "Adiprene".
The intermediate blanket of Dolcimascolo et al, also known as the relaxable layer, is preferably formed of a polyurethane rubber about 0.25 inch in thickness having sufficient resiliency to allow the roll to deform when brought into moving contact with the photoconductive drum surface to provide an extended contact region in which the toner particles can be transferred between the contacting bodies. However, the above-described biasable or bias transfer members are sensitive to changes in relative humidity. The heart of the bias transfer member is the thick resilient blanket or relaxable layer having a bulk resistivity falling in a well-defined operating range selected in relation to roll diameter and surface velocity. For these prior art systems the bulk resistivity of the blanket can vary over the range from about 10.sup.7 to about 10.sup.11 ohm cm. A variation in this resistivity of about two orders of magnitude, primarily as a result of static and dynamic changes in relative humidity, RH, (extending generally from 5 to 10 percent RH to 85 to 100 percent RH), is observed for the practical available commerical materials in this resistivity range. This variation in resistivity due to relative humidity effects the quality of image transferred from the photoconductor to the final support material. For this reason, it is desirable to provide biasable transfer members which are not effected by changes in relative humidity.
The polyurethane materials which are useful in forming the thick resilient blankets often have resistivities which are outside the acceptable resistivity limits for bias transfer members, i.e., about 10.sup.7 to about 5.0 .times. 10.sup.11 ohm cm. Accordingly, it is desirable to utilize additives to control the resistivities of the elastomeric polyurethanes used in the manufacture of biasable transfer members. Normally, the resistivities of the elastomeric polyurethanes are too high for use in biasable transfer members, and they must be adjusted from the higher values to a suitable range, generally from about 10.sup.7 to about 5.0 .times. 10.sup.11 ohm cm, so that the elastomeric polyurethane will transmit the bias potential from the substrate upon which it is coated, to the outer periphery of the elastomeric polyurethane.